Kozue is first seen in the episode The Sunlit Garden: Prelude. In the episode, most of Kozue's backstory is also revealed. Kozue once loved playing the piano with her twin brother Miki, and the two were extremely close. Kozue was a talented pianist, but together, she and her brother shone. Their parents wanted them both to be successful in music, even going so far as to schedule a tour for the twin prodigies.

Unfortunately, Miki fell ill before their tour began, forcing Kozue to continue the show alone. Nervous and uncomfortable without Miki, she refused to perform, and gave up piano soon after, distancing herself from her brother with whom she was constantly compared. Kozue's relationship with her parents was never the same after that.

With her brother in the student council and already in high school though he is of middle school age, it's clear Kozue lives in Miki's shadow, a fate she quite dislikes. However she still deeply cares for and loves Miki, and reveals in the Black Rose saga that she purposefully dates men she knows he will disapprove of because she enjoys having him worry about her. It is Miki's growing infatuation with Anthy that makes her become a Black Rose duelist, as she says she can feel him "slowly fading away" towards Anthy. She is extremely protective of Miki, going as far as to push a music teacher she feels is a threat to him down the stairs.

Kozue's sexuality is a large aspect of her character, as she uses her attractiveness to achieve her goals. She is implied to be sexually active with multiple men, including Touga, and she brags to the other girls about her prowess saying she "doesn't change boyfriends" she "just adds on more". It is also implied that she has feelings for a female friend of hers, and in the Sega Saturn game, she is shown having a sexual relationship with Sanjouin Chigusa. She even exhibits sexual attraction to Miki himself, as she had stroked his chest and attempted to kiss him while under the control of the black duelist ring.